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Cheiroballistra arms
= Introduction = While the generic form of arms is transmitted clearly from the original text and manuscript diagrams, there are a few important details that require discussion and real-life testing. Look here for closely related articles: * Description of the arms in the original manuscript * Making steel hooks for arms * Pages in "Arms" category = Materials = Everyone seems to agree that the hooks of the arms were made from iron/steel. Most agree that the cones were of hardwood. However, Wilkins' (1995: 33) bad experience with breaking hardwood cones led him to start using bronze cones instead. That said, Wilkins' had increased the diameter of the springs (1995: 23-24) arbitrarily, because he assumed the cheiroballistra to be a winched weapon. He also lengthened the metal hooks far beyond the cones (1995: 33). Both of these increased the strain on the cones tremendously, so it's no wonder they broke in his tests. This was noted earlier by Iriarte (2000: 59). If, for sake of the argument, we assume that a more durable material than wood was required for the cones, why make an elaborate composite construction in the first place? It would have been a lot simpler and cheaper to just forge the entire arm from steel and skip all the dovetail, hoop and tenon nonsense. We can therefore safely assume that wood was used for the cones and steel for the hooks. If these do not work in practice there must be something wrong in the reconstruction. = Length of the bars = First ambiguity in the text is the length of the (metal) bars of the arms, which is not given. The codex M version of the diagram (see Schneider 1906: 162; Wilkins 1995: 32) show the bars extending quite far beyond the end of the cones, in the codex V version not so much (see Wilkins 1995: 32). Due the above ambiguity scholars such as Marsden (1971: 226-227) and Wilkins (1995; 2003) were able to make the metal hooks very long without violating any instructions given in the text. Wilkins had do this because he had issues with too short draw length, which in turn were caused by placing of the little ladder too close to the front of the case. From engineering point of view extending the metal bars significantly beyond the cones is very unwise for two reasons, each of which "feeds" the other and amplifies the problem. First, extending them places too much stress on the ends of the metal hooks, which would have otherwise been supported by the rigid wooden arm. In other words, they become a lot more prone to bending and therefore they have to be made much thicker and thus heavier, which indirectly weakens the ends of the wooden cones whose dimensions are known from the manuscript. Also, as the arms are only a method by which energy is transmitted to the bowstring and to the bolt, they should be as light as possible. The heavier the ballista arms are, the more energy they waste, especially if their forward momentum is stopped by the pads in the curved field-frame bars and not the bowstring, which would otherwise transfer some of the wasted energy into the bolt. Second, as Iriarte (2000: 59) points out, extending the bars far beoynd the cone adds shearing stress to it's end, which is already it's weakest point. This probably requires further explanation. Imagine the bowstring trying to pull the bar at 90 degree angle away from the cone: the hoop gets pulled upwards, and the end of cone gets pushed downwards. So, all force applied to the bar is concentrated to these two points and the middle part of the bar is free to rise upwards. Extending the bars far beyond the cones increases the pressure experienced by the two support points, very likely resulting in cone breakage at it's narrow end if too much force is applied. = Shape of the channel = Shape of the channel in the cones is also a matter of debate. On the other hand, in all versions of manuscript diagrams (M and V) the channel has a distinct dovetail shape. However, as mentioned on the translation of cheiroballistra page and also by Iriarte (2000: 59) P.H. talks about a square or a rectangular channel. So, looking solely at the cheiroballistra text either interpretation would be acceptable. However, there are technical reasons why a rectangular channel makes more sense. First, it is much quicker and easier to make and an extremely tight fit is very easy to accomplish. A good dovetailed channel can be made with some effort by attaching two strips of wood with correct angles to the cone proper. However, the angles of the strips have to match those of the bar very closely. Similarly, forging a rectangular bar is trivial, whereas forging a dovetailed one without a special form is difficult. Filing the bar to the correct dovetail shape works, but takes a lot of time. So, in a nutshell, a tightly fitting dovetailed channel requires lots of time and care. There also an another reason why a dovetailed joint does not make sense: as discussed above, the middle part of the bars rises upwards when the hook is pulled by the bowstring, effectively ripping apart the wood fibers. This could result in breakage when the sides of the channels give up. There is also the option that the bars were rectangular in cross-section, but became progressively narrower towards the hooks. Forging a tapering, rectangular bar is fairly easy and if the channel is made to match it exactly, problem #2 (below) gets solved as a bonus. The tapering channel and bars allow making light and fast arms, which will also be strong. It will require some extra work compared to a simple rectangular bar and channel and may not be worth the effort. = Attaching the bars to cones = The manuscript text says that the channel for the bar runs the entire length of the cone. However, it is also clear from manuscript text and diagrams that the bar did not fill the channel in the cone entirely; instead, it stopped at the middle, apparently just before the torsion spring. At this point the cone and the bar were joined using a hoop of some sort. Another attachment point (a socket?) was right at the tip of the cone. As mentioned earlier, the cones taper towards the tips - this has important implications as we'll see later on. All of this complicates the interpretation of the arms somewhat, as optimally we'd need something that works in practice yet does not require changes to the given instructions. In fact, we have several problems to solve: # What material was used for the hoop near the base of the cone? # How do we keep the bar from sliding towards the (tapering) tip of the cone? # How do we prevent the straight end of the bar from rising up from the channel? # How do we support the tip of the cone, given that the text does not help us much? The hoop is usually (e.g. Marsden 1971: 227) assumed to be of steel. This is probably because of the wording in the text, which gives the impression that the bar and the hoop are united before they're inserted in the channel in the cone. This arrangement attempts to solve problems #2 and #3. However, using a separate hoop would be much simpler and as strong, provided the bar is turned upwards at the edge of the hoop, so that it's prevented from sliding forward. In this case, the hoop itself only solves problem #3 and we need to adapt the arm slightly to solve #2. Whether the bar is united with the hoop or not, we can make it closed or open, the latter meaning that the hoop is tightened after insertion into the cone. With a closed loop approach we soon run into trouble: even if we manage to make the fit tight initially, the hoop will eventually loosen up and let the bar slide forward, which is potentially very dangerous. If we decide to use an open hoop we can prevent bar's forward movement by filing a shallow groove for the hoop into the cone and squeezing the hoop tightly into it. This has the downside that cones become somewhat weaker at this point. If the groove is very low, this should be acceptable, though. This "open hoop" approach sounds more probable, if we have choose between the two. All this said, we can also solve each of the problems (1-4) listed above separately to create a better-quality arms with least amount of effort. Although it contradicts the text somewhat, it would be easiest to make the bars extend beyond the base of the cone and bend their ends into L-shape, i.e. against the base of the cone. This solves problem #2 with least effort, and focus on the remaining separately.In fact, problem #3 is already partially solved as the pressure of the torsion spring will prevent the bar from rising from it's channel. However, the hoop is still helpful when the arm is removed from the spring. Probably the best material for making the hoops is sinew thread, which is extremely strong, light, abrasion resistant and tightens when it dries. So far nobody has figured out what socket at the end of the cone was like. I think Iriarte (2000: 59) is by far closest when suggesting that the cone and the bar are simply wrapped together with a cord. However, he made (2000: 60) the ends of the cones protude from the cones as tenons, thus weakening the cone's tip. As discussed here, the term τόρμος describing this part means "any hole or socket, in which a pin or peg is stuck". So, there's no need to add a tenon; instead, we want a socket, which we can form by adding a second hoop at the end of the cone. So, end of the cone becomes the socket, into which the bar is pushed. This interpretation finds support from the manuscript diagrams, too. The best material is probably - as with the hoops - sinew thread. = Summary = As the earlier discussion was rather lengthy, here are two options for making the arms. Arms that follow the manuscript as closely as possible * Rectangular groove in the cone * Bar bent upward at hoop * Hoop open and sunk into the cone * Socket made from sinew thread (or a metal hoop) Arms that are simple and strong * Rectangular groove in the cone * Bar bent around the base of the cone * Hoop made from sinew thread * Socket made from sinew thread Category:Backup Category:Cheiroballistra Category:Practical Category:Theoretical